Piezo-electric resonator



' July 2, 17929. w. G. CADY Re. 17,355

y `PIEZO ELECTRIC RESONATOR Original Filed Jan. 28. 1920 2o '77 A70 7 f /fl .6. b

/G y l i Arrow/5x l .vf-7.6.2. i fm f if AAo insitelyrcharged sheets o l 5o coatings, it tends to become deformed. It

l. Ressued July 2, 1929-. y

UNITED STATES- WALTER G. CADY, OIE MIDDLETOWN', CONNECTlCUT, `ASSIGrNOR TO RADIO `CORPORA- TION OF AMERICA, A CORPORATION OF DELAWARE. A

II'EZO-ELEGTBIC RESONATOR.

Original No. 1,450,246, dated April 3, 1923, Serial No. 354,659, led January 28, 1920. vApplication. for

reissue :nea apra 2, 192,5.4 serial No. 20,277.

I DIVISION A.

tricity. In the specification and the claims,

the action of the electric forces to cause rne- -chanical deformation will, Afor brevity, be 1 5calle`d stimulation; and theV development of the electroinotiveforce by the deformation will be called response. A large number of bothnatural and artificial crystals have been found to possess this piezo-electric property. The general knowledgevon the subject has A'so far developed that it is now possible to predict in advance whether a given crystal Willshow the piezo-electric effect, and in what manner a plate or'rod should be out from 25 the crystal in order to exhibit this effect to the reatest possible degree. s

, T e crystal that has been most commonly employed in piezo-electric experiments, in the past, has been quartz. As a rule,- investigators have used thin, flat plates or rods of quartz rectangular in cross section, and coated on their opposite faces with tinfoil. These plates or rods have beensocut from the crystal that the direction of their length has been perpendicular to the. opticalA axis and to one of the binary or 'electrical aires of the crystal; the direction` of their breadth,y

`parallel vto the' optical axis; and the direction of their thickness parallel the said electrical axis.l Al more complete description may be found in my paper in. the `Proceedings of the Institute of Radio En gineers,'vol. 10, April, 1922,' beginningon page 83, and more particularly atvpage 85.

tric .field perpendicular tol its fiat faces, or `parallel to the direction of the thickness of the plate, as, for exam le, between two oppogtinfoil or other metal When such a plate or rod is placed in an elec-` tends to become elongated or shortened in a dlrection perpendicular to the electric field,

and parallel to its length; and the thickness `the'plate is an elongation or a shortening in its length, or a decrease or increase in its thickness, depends upon the polarity of the electric field. ormations of thequartz plate,'in directions perpendicular and parallel to |the electric field have been known in the prior art under the terms transverse and longitudinal effects respectively. lVhen such plates are mechanically deformed, on the other hand, as when' they are compressed endwis'e, one of the metalV coatings becomes charged positively, and the other negatively, the signs of the charges having'a definite relation 'to the 'principal 70. axes of, the crystal from Whichl the plate is cut.

More recently, investigators, including my' self, have experimented on the effects produced when the crystal plates, instead of being charged statically, are subjected to alternating electric fields of various `frequencies. Nicolson, for example, has published a. paper on the use of Rochelle-salt,crystalslas receivers and transmitters in telephony.. F olvlowing this line of investigation, I have conducted' experiments with different crystals exposed to the effect of higher frer uencics, such -1 as are used in radio-telegrap y, and have studied,l particularly, the periodic g5 lengthening and shortening oflrectangular plates when the coatings were connectedto a source of high-frequency electro-mot1ve force -and the invention upon which this application for Letters Patent is based has been the result of this study and the informationA 3 obtained from such experiments.

It is evident that a crystal plate, with conductive coat-ings, as above described, constitutes a trne electric condenser, and possesses a. certain electrostatic capacity. Ifhave observed that the capacity and the resistance of such a plate varied under dilferentcircumstances, and for this I have ascertained the cause and developed the mathematical theory.

These two piezoselectric def- 60- transverse sind the lon itudinal effects.

'wasmentioned in my atent No. 1,472,583, granted Cctober 30, 1923, it will be more con- My experiments have included both tlxie S lvenient to explain and describe the invention 1n terms of the transverse effectybut the explanations and descriptions apply equally Well to'piezo-electric crystals, or portions or preparations thereof, in'which the longitudinal effect or any other type of piezo-electric deformation is utilized, The late or rod may, for example, be so designe and mounted as to permit taking advantage of the prin- 'ciple of flexural vibrations. Not all crystals have both the longitudinal and the transverse deformations. Rochelle salt, for example, belongs to a crystal class such that the lon gitudinal eli'ect is absent. Y

The phenomenon may be `thus briefly ex- Y plained in terms of a transverse effect. A rectangular plate, likeany elastic solid, is capa- 4 ble` of being set into longitudinl vibrations.

If suddenly struck on one end, the vibrations i are free and of a frequency dependent essentially upon the elastic modulus, the density of the material of which the plates is composed,

and the length of the plate. The equation for such longitudinal vibrations is well known and may be found on page 89, line 2, of my said paper. This periodic elongation if and contraction of the piezo-elect'ric plate will, as

explained above, cause periodic positive andl negative charges, at the same frequency, to appear on the metal coatings, thus giving use to an alternating electromotive force which may be observed and utilized in various Ways'.

If,'ho'wever, the vibrations are impressed on the plate by some outside agency, "as occurs in these piezo-electric experiments, they are said to be forced, as distinguished yfrom free. The

vone less than the natural to one higher, the amplitude of vibration will be at first small,

will rise to a large; value as "the resonant frequency isreached and will .then fall again.

In further illustration, assume a plate of piezo-electric crystal having, on its 'opposite faces, tinfoil coatings connected to a source of high-frequency elettromotive force. As the plate vibrates, its length increases and diminishes alternately, and although such changes in length are so minute as to escape detection under ordinary conditionsthey are suflicient to exert a marked reaction upon the electric current. The current is'mo'dified by this reaction both in phase and amount. The reaction isof two kinds-assuming the source of elecy tromotive force to be-constantufirst: a ca? pacity reaction such that, on slowly increasing the frequency, the apparent parallel'capacity of the plate passes through a maximum at a i frequency slightly below resonance, followed at a frequency slightly above resonance, by a minimum Vso low that the aparent parallel capacity actually becomes negative, and returning to the normal, positive value'as the frequency is furthemincreased; and second, a

4resistance reaction, slichhthat, owing to the stances, directly upon itsvvidth inversely i upon its thickness, 1n accordance with Well` j* v known piezoaslectric, mathematical equations." It is this reaction that I utilize Yin my invention. lf the crystal plate is connected to a source of constant alternating voltage, an ammeter connected in series with the coatings'vv'ill be found to pass through a maximum at a frequency slightly below the resonant frequency, followed by a minimum at a frequency slightly above the resonant frequency. A somewhat different result may be obtained by connecting the coatings in parallel with the capacity and the self-inductance of a tuned electric circuit, assuming still that the electromotive force impressed on thecoil is of constant magnitude, but variable frequency. The current flowing to the coatings will, as in the previous c ase, pass through a maximum, followed by a mm1- vmum, as the frequency increases; but if the resistance of the circuit falls Within-certain limits, such as are quite commonly employed, 1n

practice, a secondary maximum and a secondaryminimum willbe found, lying between the first-named maximum and minlmum. AIt 1S unnecessary,forthe present purpose, to'- explain the theory of these' reactions, as 1t 1s fully explained in my above-referred-to paper 11o inthe Proceeding of the Institute of Radio Engineers,`with particular reference to the curves I and i in Fig. 6, page 101.

When the plate is made from a piezo-electric crystal of good elasticfqualities, such as i quartz, When its Width is small in comparison with its length', and it 1s mounted 1n such manner that its vibrationswill be damped as' little as possible, the reaction is extremely sharp and very pronounced. Such a plate, 120 connected to -a source of electric oscillations of variable fre uency, will respond when one and onlyone requency-neglecting for the moment overtones-is being generated. It forms a piezo-electric resonator, somewhat,125 analogous tothe acoustical resonators of Helmholtz. I fthe natural frequency of the plate has been'determined by comparison with a standard wave-meter, or otherwise,lit mayI be, in turn, used asastandardQfOr Calibrating au itself. In fact, it is within the scope of.

`raam:

radio and other high-frequency circuits, and a number of such plates, of different lengths, may be used for calibrating a high-frequency circuit over as Wide a range as desired.

It is not necessary that the device comprise a long, narrow plate, although this has its advantages in permitting greater sharpness of tuning, and in securing greater freedom from effects that might be produced byundeslred modes of vibration. The plate may have various shapes, or some other form than 'a plate may be used, including thelentire native crysthe present invention to use other electromechanical vibrators than pieno-electric crystais. The only essential cond'ltion '1s that the -vibrator shall be mounted and connected to an oscillating circuit of variable frequency in such a manner as to react electrically upon such circuit ata particular frequency. In general, the more complicated the form, the more numerous are the frequencies to which the unit will respond.

In the accompanying drawings, I .have

illustrated graphically the nature of my in: vention and various Ways of modifying the same and applying it to practical use; and .to these drawings IV now refer.

Fig. 1 is a diagrammatic illustration of the instrument in its simplest form.

Fig. 2 shows, in a similar manner, a modification of the same.

Figs 3l to 8, inclusive, are diagrams illus trating usesand applications ofthe invention which will be described in detail. In Fig. 1, the piezo-electric plate is desig` nated by the numeral 1. The conductivecoatings2 are` connected with a source of high-frequency current 4 by Wires 3 and an ammeter 5 is shown in this circuit.

` If the frequency be increased, the current i traversing the circuit, as measured by the ammeter 5, will bev found to' pass through a maximum at a frequency slightly below the -electric plate or plates may,

of the crystal when the'erystal connected directly ,in the circuit, but the frequency at which tlie above-mentioned minimum'occurs is so close to the natural frequency that, for most practical purposes,l it may he regarded as the natural frequency of the crystal.

These results follow readily from`Fig. 11 of.-

my before-mentioned paper and actually do", occur, in practice.

In Fig. 2, a modification ofthis instrument l is shown. When the desired frequency is relatively low, `crystals of suilicient size may.,

not be economically secured, in which case, I use a thin rod 6 of any -solid substance of goofh elastic properties, Afor example, steel, asthe vibrating element. lItshould be of such length that its natural period of vibration, calculable frmuthe Well known equation, is of the desired value. This rod is set into vibration by means of arpiezo-electric plate 1 of relatively small dimensions, cemented or otherwise held in intimate contact with it. The shape, size and material of the plate 1 'should be such as to excite Vas strong, longi tudinal vibration in the rod 6 as possible,

when the metallic coatings 2 are connected tov source of high-frequency electro-motive orce, but, in any event, it should be small 'enough so that none of its ,'`wn modes of vibration, to which it may be piezo-electrically excited, are of a frequency suiiiciently near the naturaly frequency of the rod 6 to be troublesome.- The natural frequency of the rod 6, is, of course, slightly modified by the' plate 1 attached thereto, but the combination of the two forms affunit of very constant frequency, capable of reacting sharplyl on a high-frequency circuit in exactly the same manner as the" lates when used alone, as in the preceding gure. l

It is not, of course, essential that the exciting plate of piezo-electric crystal be attached to the rod 6 atthe end of thelatter. A piezofor example, be

resonant frequency, :followed by a minimun@v cemented to one or both sides of the rod 6,

at a frequency slightly frequency. If the frequency be "decreased, this process will be reversed. This is true, irrespective of the nature of source 4, whether a vacuum. tube, or of some other character, assuming, however, that the source is of constant,.alternating voltage. If a. vacuum-tube generator, with the usual associatedcircuits, is used as the sourceof voltage, -a distinction must be made according to Whether the crystal is connected directly in the generating circuit, or in a secondary circuit, in which latter case, the source 4,. in Fig. 1, might be a coil in which electromotive force is induced from the generating circuit.` This latter case is further discussed below. The reactions ofthe crystal `are such that, with a tube-generating circuit, as commonly employed, it isusually impossible to make thecircuit oscillate at 4exactly the natural frequency of vibration though solid materials above the resonantb preferably at the center ofthe rod 6. Nor is The rod 6 should be of such material'that' its natural frequency is as slightly as possible affected by changes of temperature, but, in

this. It need not be of solid material,-

will probablybe found besa-for it is possible to use, in place of a solid, a column of liquid, for example, mercury, in a long narrow'tube, one end of t-he column being in contact with the piezo-electric plate or crystal.

of the length of the column of fluid.

The piezo-electric resonator may be usedv in variousways, as, for example,

@ne advantage of this varrangement 1s the possibility of varying the frequency ofvibration at Will, by adjustment to produce a large impedance or reactance 1n an alterany case, correction may readily be made for nating circuit at-a certain particular frequency or frequencies, to serve as a standard of frequency or Wave length in high-frequency circuits, or, even, for such purposes as coupling one high-frpqueney circuit to another, in order totransmit energy from one to tlie'other circuit at a certain particular intended inerely to illustrate the general prinand 16 connect the condenser 14 with the re-l ciples` .,which apply to any of the numerous types of-.vhigh-frequency circuit.4 s

Inthis case,the following action takes place: Assuminggtfli'at 3 represents the primary circuit, thenggitrhenever -an alternating current, at the critical frequency, flows in that circuit, the plate 1 Will be brought into energetic vibration through the agency ofthe altern ating potential di'erences between the coatings 7. These vibrations, will, in turn, generate potential differences in the coatings 8, which will cause an alternating current of the same frequency to flow in the secondary circuit 11. At other frequencies, the forced vibrations in the resonator 1 will be of very small amplitudc, hence the induced current will be correspondingly small. thecoil 12, at the frequency of the crystal, may be detected in any desired Way, as by means of a telephone receiver 19, as shown in Figs. 5, 6 and 8, or it may be otherwise utilized.

When it is desired to reduce the current in a hi gh-frequcncy circuit to as low al value as possible, at a critical frequency, the arrange-A ment shown in Fig. 4 niay be employed. In this figure, the piezo-electric resonatorl is connected in parallel with a .condenser 14, which may forni part of any tuned oscillatory circuit in which alternating current of vu- -riable frequency is owing.- The wires 15 mainder of the circuit, through an aninieter 5. At a critical frequency, the absorption of energy Vin the pie.o-electric resonator causes the current in thxgameter to. pass through a.

minimum.. This decrease in current the greater, the smaller tliecapacity of the condenser Witli respect to that of the resonator.

As has been explained above, the'current flowing to the crystal body does not sink to a simple minimum at resonance, but as the frequency is increased, it passes through a maximum, followed by a minimum, with a -possible secondary maximum and minimum l infirm-between.

For this reason, when it is desired to reduce .a current to a minimum at The current flowing in a predetermined' frequency, the current in question should be either that through the amineter 5 in Fig. 4, or else that flowing to condenser 14 in the same figure, since in the latter case the current falls in general 4to a simple minimum at resonance. Similarly, in Fig. 7, the currentl passes through a minimuni at resonance in the ainmeter 21, and also in the condenser 10.l This is explained in my said paper in the Proceedings of the Institute of Radio Engineers,.pages 99 to 101.

This' circuit constitutes a piezo-electric v v iltering circuit, and, as is Well known to be the case with all narrowband filtering circuits, it is an advantage to have the electrical resistance as low as possible.

This ligure also indicates one way-in which the piezo-electric resonator may be made to serve as a standard of frequency or of wave length in radio-telegraphy It is only necessary to connect several such resonators, in succession, in placel of that shown, each time :making note of the readings of the condenser or other apparatus at the critical frequency.

capacity, of a few niicro-inicro-farads, in se-y ries with the resonator. Often the response is suiliciently strong, even when one orvboth of the terminals of the resonator is entirely disconnected, the capacities between the resonator terminals `and thecircuit being suiiicientto effect aperceptible response. By means of these capacities, the resonator is enabled to absorb enough energy to niakerit vibrate and to react perceptibly upon the circuit.

In Fig. 5, the' piezo-electric resonator 1 forms part of an oscillatory circuit loosely coupled to a tuned circuit comprising a coil 17 and a condenser 10.. In parallel with the latter, is a detector 18 and a telephone receiver 19, or some other indicating device maybe properly connected With the current. When ythe resonator 'is connected in parallel with the coil 20 in thefirst circuit and the frequency of the alternating current is varied through the critical value, the sudden ieaction ofthe resonator upon the rcurrent in 2O produces an audible click in the' telephone 19. It is not generally necesary that the circuit of coil 17 should be in exact tune With that containing 20. i Fig. 6 illustrates the same plan as the preceding figure, but, in this case, the piezoelec- The alternating currentL A resonator, when used as a frequency Vtric resonator is in parallel with the conas upon the circuit comprising the coil v20. 'This connection of the resonator in parallel with the -capacity in a secondary circuit is found, in practice to be the best, Whether the circuit isused for the standardizing of frequency or as a filter. The timing condenserin parallel with the crystal and the resistance of the circuit should haveas lowV values as Fig. 7 is similar to Figs. 5 and 6, except, that, instead of the detector and the' telephone, there is shown a high-frequency arn- Ineter 21. Byvarying the frequency of the current in coil 20, and keeping the secondary circuit, which comprises the coil 17 and a variable condenser 10, in electrical resonance.

with the currentl in 20,\it is possible to observe quantitatively the manner in vwhich the current in the ammeter passes through a minimum at the critical frequency, and to deter mine the settings of condensers and other in strurnents corresponding to. this frequency,

with a high degree of precision.

For many purposes, it is immaterial Whether the crystal is in the oscillatory circuit comprising the coil 20, as shown in Fig.

5, or in the circuit comprising the coil 17, asin Figs. 6 and 7. When the highest precision is sought, however, it is better to include the crystalin adiferent circuit from that which it is desired to standardize. With the crystal connected as in Fig. 5 or 6, the operation of timing the generating circuit comprising the coil 20,`so as to bring it into synchronisin with the natural frequency of the resonator, is as follows When the frequency of the current in the coil 20 passes throughthe critical value, the crystal is set'into vibration, and it continues lvibrating for a fraction of a second even after theirequency ofthe impressed al# ternating current has assumed a different value. The crystal, by itsvibrations, generates a pieZo-electroinotive force, and two fre# quencies arerthen present in the secondary circuit'zwone, that the impressed' alternating current as deter ed by the electrical constants ofthe circuit; and the other, that of the generating crystal. If the frequencies of these two currents are near each other, they may differ, say, to theV extent of a few huny dred cycles, the currents will combine, accordc ing to the heterodyne principle, 'toproduce a beat-note of musical quality, which may be detected in the telephone receiver19 as the before-mentioned'click. The pitch and duration of the beat note depend upon the eX- tent to which the condenser 10 is varied after ,p the resonator has been set into vbration,and

also upon the rate at which the vibrations of the resonator die away. Iftliis rate is very Y rapid, then, on varying'- the condenser 10 through the critical value, only a short click,

instead of a beat note of recognizable pitchv Willbe heard. ','lhis phenomenon is described in "my paper above-mentioned, particularly page 109. 1 lVlien the resonator is used as a frequency standard, connected to a secondar circuit as in Figs. 4, 6, or .7, the indicating evice is, as

a rule,'eitlier acurrent-measuring device in `tl1e main circuit, like the ainineter in Figs.

ple iiiiimuni at the natural frequency of the resonator. These facts are explained more at length in mysaid paper, particularly in connection with Fig, 5 ofthat paper and on page 101'.

Fig.V 8 represents a form of circuit in comnion use for generating high-frequency currents by means of a three-elemeiit vacuum tube 22. In this figure 23 and 24 represent, respectively, the\bat-tery and the regulating resistance in the filament circuit; and 25 is the plate battery. A telephone receiver 19 has a fixed capacity27 in parallel with it and 2G is the feedback coil, which is coupled to coil 28 in the grid circuit. Condenser 10, in

parallel with coil 2S, is `used to control the frequency. The piezo-electric resonator is in parallelwith the condenser 10 and its capacity isso small as not to introduce a perceptible error.4

*Currents of varying frequency may be ob-/f cause a beat note to be heard in the receiver 19. In many cases, moreover, it is possible to detect the` resonant frequency With a veryV loose couplingof the resonator, as when only one side of the resonator is connected, vor evenv when both sides are v,disconnected as above described'. y In 'such cases, of course, the crystal is nevertheless connected with the circuit through the minute capacities between' igiothe resonator terminals and adjacent pori In the description and-'illustrations given above, Ihave assumed that a supply of undain-ped alternating current was available. While the", best results are obtained in this way, it is possible'to use the resonator as a standard .of wave length also when damped waves only are available, as, for` example, from a buzzer circuit. The reaction is then much less. pronounced, owing to the fact that a damped train of Waves contains, not a single frequency, but a combination of many frequencies. If, however, damped waves must. be used, it is best to'make the decrement as small as possible, for example, by the use of impact excitation. Some of the circuits above described, asfor example, the circuits shown in Figs. 3 and 6, or proper modiications of them, may then be employed.

From the nature of the invention thus above described certain modifications are obviously possible. "For example, to secure piezo-electric resonators of various frequencies, weights may beA attached to .one or both ends of a piezo-electric plate, which will have the effect of lowering the frequency.

I have referred above to overtone frequencies, but in this connection little need be added. lVhile the fundamental 'frequency of the plate or rod will usually give the strongest reaction, the various overtone vibrations can also be employed, giving reactions at frequenciesapproximately two, three or more times the fundamental. 0r, if the plate has other dimensions comparable with its length,

or vif some other shape than that of a plateis used some other mode of vibration, giving a stillr different frequency, may be utilized. When the vibrating unit has once been suitably prepared andmounted, its resonant f-requencies are, .to a high degree of precision, fixed for all time. p

What Iclaim as my invention is y l. The combination4 with an alternatingcurrent circuit of high frequency, of a means -.forreducing'the flow of current therein at any-particular frequency comprising a body of piezo-electric character with conductive coatings over the regions thereof which exhibit opposite electrification, and connected with the alternating-current circuit, the said body being so designed that its natural frequency of vibration will be in mechanical resonance with the said particular frequency of the alternating current;

, 2. `The combination with an alternatingcurrent circuit of .high frequency, of a means for reducing the flow of current therein at any particular frequency, comprising a body of piezo-electric character with,conductive coatings 'over the regions thereof'which exhibit opposite electrification and connected with alternating-current circuit, the 'said body being sodesigned and mounted that its mechanical vibrations Will be damped to the least poible extent and so that its natural frequency of vibration Will be in mechanical resonance with the said particular frequency of the alternating current. 3. The combination with a tuned alternating-current circuit, of a means forV reducing the' flow of current therein at any particular Y frequency withwhich the circuit is in electrical resonance, comprising a body of piezoelectric character 'with conductive coatings over the regions thereof which exhibit opposite electrilication, and connected in parallel With the capacity or inductance of said alternating-current circuit, the said body being so designed that its natural frequency of vibration will be" in mechanical resonancel with the said particular frequency of the al. ternating current.

4. The combinationwith a tuned alternating-current circuit, vof a means for reducing the flow of current therein 'at any particular frequency with which the circuit is in electrical resonance, compr1s1ng a body of piezoelectric character with conductive coatings nected with the receiving circuit and so designed that a natural frequency of'mechanical vibration ofthe body shall be substantially equal to a predetermined frequency of the current flowing in the receiving circuit.

6. An alternating-current system comprising a `receiving circuit in lwhich currents of variable frequency ar'e adapted to be received, and a piezo-electric body connected lwith the receiving circuit and so designed that a natural frequency of mechanical vil bration of the body shall be substantially equal to a predetermined frequency of the current iowing in the receiving circuit, the body being so mounted that its mechanical vibrations shall be damped to substantially the least possible extent.

7. An alternating-current receiving system comprising an alternating-current circuit and a piezo-electric body loosely coupled to the circuit. n

8. An alternating-current system comprising an alternating-current circuit havlng a variable reactance, whereby the system mayv be tuned, a piezo-electric body, the body `-being connected in parallel with the reactancc and being so designed that a natural frequency of mechanical vibration of the body shall be substantially equal to the frequency of the current.

9. An alternating-current system comprising an alternating-current circuit having a variable reactance, whereby the system may be tuned, a piezo-electric body connected in parallel with the reactanee and being so deprsing an alternating-current Vcircuit and a signed that a natural frequency-of meehanipiezo-electric body adapted tovibrate both 10 cal vibration of the body shall be substanby reason of the transverse and the longitially equal to the frequency of the currentf tudinal effect.

and the body being :so mounted that its In testimony whereof, I hereunto aix my mechanical vibrations shall be damped to signature.

substantially the least possible extent.

10. An altemating-current system com- WALTER G. CDY. 

